Fiber optic communication systems typically employ wavelength division multiplexing (WDM), which is a technique for using an optical fiber to carry many spectrally separated independent optical channels. In a wavelength domain, the optical channels are centered on separate channel wavelengths which in dense WDM (WDM) systems are typically spaced apart by 25, 50, 100 or 200 GHz. Information content carried by an optical channel is spread over a finite wavelength band, which is typically narrower than the spacing between channels.
Optical channel monitoring is increasingly being used by telecommunications carriers and multi-service operators of fiber optic systems. As the traffic on optical networks increases, monitoring and management of the networks become increasingly important issues. To monitor the network, the spectral characteristics of the composite signal at particular points in the network must be determined and analyzed. This information may then be used to optimize the performance of the network. Optical channel monitoring is particularly important for modern optical networks that use reconfigurable and self-managed fiber-optic networks.
For example, reconfigurable optical add/drop multiplexers (ROADMs) and optical cross connects, which are used to manipulate individual wavelength channels as they are transmitted along the network, require an optical channel monitor. A ROADM allows dynamic and reconfigurable selection of wavelength channels that are to be added or dropped at intermediate nodes along the network. In a ROADM, for instance, an optical channel monitor can provide an inventory of incoming channels as well as an inventory of outgoing channels and to provide channel-power information to variable optical attenuator (VOA) control electronics so that the power of added channels can be equalized with the pass-through channels.
Ideally, monitoring of an optical communications system should be based on the analysis of the actual optical signal waveform across the entire range of wavelengths of interest. Spectral analysis of this type can be performed using a variety of known signal and spectrum analysis equipment. For example, optical signal analyzers are known for determining characteristics of an optical signal such as, for example, power level, enter wavelength, extinction ratio, eye opening, signal-to-noise ratio, polarization dependent loss (PDL), dispersion etc.
A number of different devices have traditionally been used as optical channel monitors. For instance, in order to monitor respective channels of a WDM optical communications system a Parallel Detection Monitor (PDM) can be used to determine average and peak power levels, as a function of wavelength, across any desired range of wavelengths. The PDM uses a grating or other device to split or demultiplex a WDM signal into its individual channels or wavelengths and a series of photodetectors to measure the power in each channel or wavelength. Alternatively, a Tunable Filter Monitor (TFM) may be implemented using a fixed filter bandwidth, center wavelength tunable optical filter together with an optical detector. Such a filter transmits or reflects only a narrow portion of the transmission band, which can then be detected by the detector. By scanning the filter center wavelength across the entire transmission band, a full spectral picture of the transmission band can be obtained, and the channel information (e.g., wavelength, power, OSNR) can be extracted using suitable signal processing methods. Tunable filters can be implemented using for example thin film technology to create a Fabry-Perot type filter, with the tuning accomplished either by temperature effects, using liquid crystals, or by tuning the angle at which the signal is incident on the filter. Other technologies for implementing tunable filters include, but are not limited to, tunable Bragg gratings and filters based on the acousto-optic effect.
While the above-described devices enable some degree of optical channel monitoring, they tend to suffer from a number of disadvantages. In particular, these devices need to be calibrated for a signal having a particular fixed channel plan. The channel plan may specify a number of parameters such as the center frequency and bandwidth of each channel, the spacing between channels, a power level of each channel and the modulation format used by each channel. In the case of an PDM, the channel spacing and the filtering characteristics such as the bandwidth of each channel must both be selected in advance. While a channel monitor using a tunable filter has the ability to scan across the wavelengths, thus in principle allowing a channel plan with irregular channel spacings to be monitored, such a monitor still requires advance knowledge of the channel bandwidth and modulation format. Therefore these devices cannot be designed to be independent of the network's channel plan.